Bring a large pot of salted water to a boil. Add potatoes and cook for 5-6 minutes, until almost tender.

Drain potatoes and place in a lightly greased 9×9" baking dish. Preheat the oven to 375 degrees F.

In a medium skillet, melt butter over medium heat and cook onion until softened and beginning to brown.

Stir in garlic, Italian seasoning, salt, and pepper and cook for 1 minute.

Add flour and whisk until combined.

Add milk and cook over medium heat, stirring frequently, until thickened — about 5 minutes.

Stir in sour cream and 2 cups of cheddar cheese.

Pour sauce over potatoes in baking dish, stirring to combine. Sprinkle with remaining 1 cup cheese and bake until bubbly and golden, about 20 minutes.

Remove from the oven and allow to sit for 5-10 minutes before serving.